We know it best as the story of Schrödinger’s notorious moggie&colon; a cat fated to be both dead and alive at the same time. That was an intentional absurdity, but for a photon, existence in a quantum “superposition” of different states is a fact of life. Its electric field, for example, can be simultaneously both horizontally and vertically polarised.

This is an inordinately powerful property. While the processing muscle of today’s supercomputers is limited because electron currents can only ever be on or off, a photon in a superposition represents a “quantum bit”, or qubit, that is both on and off at the same time. Alán Aspuru-Guzik of Harvard University estimates that a quantum computer of just 150 qubits would have the processing power of all today’s supercomputers combined.

A quantum computer of just 150 qubits would have the power of all today’s supercomputers